Conventionally, some dump trucks use a braking system for a service brake that functions by operating a brake pedal as a retarder when driving downhill and under other conditions. In other words, when a retarder control lever or a retarder switch is turned on, the braking system starts operating as a retarder to brake a dump truck.
When a multi-disc wet brake is adopted as such a braking system, cooling oil is supplied to absorb heat generated when braking. The cooling oil is provided by lubricating oil of a transmission that transmits power from an engine to wheels and is fed from an oil pan of the transmission via a hydraulic pump driven by the engine.
An amount of cooling oil supplied varies significantly between when the retarder is used and when the retarder is not used. Since a more than necessary amount of cooling oil increases loss of power in a braking system and reduces fuel efficiency, excessive cooling oil bypasses the braking system via a brake cooling valve (an electromagnetic proportional valve) when the retarder is not used, so that loss of power is reduced and fuel efficiency is improved (See Patent Literature 1).